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Re: [PATCH] ext2fs: Update to upstream Hurd-reserved xattr index for "gn

From: Jan Nieuwenhuizen
Subject: Re: [PATCH] ext2fs: Update to upstream Hurd-reserved xattr index for "gnu.*".
Date: Wed, 13 May 2020 14:04:04 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/26.3 (gnu/linux)

Samuel Thibault writes:

Hello Samuel,

> Jan Nieuwenhuizen, le mar. 12 mai 2020 16:12:34 +0200, a ecrit:
>> setfattr --name=gnu.translator --value='/hurd/pflocal\0' 
>> /mnt/servers/socket/1
> man setfattr says
> If the given string is enclosed in double quotes, the inner string is
> treated as text.  In that case, backslashes and double quotes have
> special meanings and need to be escaped by a preceding backslash.
> so I guess it needs 
> setfattr --name=gnu.translator --value='"/hurd/pflocal\0"' 
> /mnt/servers/socket/1
> to actually interpret \0

Yes, that's it; thank you!

So I've now managed to create a vm-image using Guix that boots the Hurd
straight into our Guile RC script, without the need to use Bash for
that.  See

    ./pre-inst-env guix system vm-image --target=i586-pc-gnu 


    wget https://lilypond.org/janneke/hurd/hurd-xattr.img.gz
    gunzip hurd-xattr.img.gz
    guix environment --ad-hoc qemu -- qemu-system-i386 -enable-kvm \
     -snapshot -hda hurd-xattr.img -m 2G \
     -device rtl8139,netdev=net0 -netdev 

So WDYT, is my patch for the Hurd (the first message in this thread)


OK to apply now?  Any ideas or suggestions on my Linux patch?

>> --8<---------------cut here---------------start------------->8---
>> fsck --yes --force /
>> fsysopts / --writable
>> mv /servers/socket/1 /servers/socket/1-linux
>> touch /servers/socket/1
>> setfattr --name=gnu.translator --value='/hurd/pflocal\0' /servers/socket/1
> Note that glibc's setxattr, i.e. _hurd_xattr_set, translates that
> into a __file_set_translator() RPC call. Did you pass the proper option
> to make ext2fs record the translator as xattr instead of passive record?


>> And I guess there must be an incompatibility between Linux and the Hurd
>> in how setfattr embeds the xattr attributes into the file system.
>> How to best "diff" this aspect in the file system; how to proceed?
> debugfs can be used for that.

Right, thanks.  I was looking for something like that.

>> Inspired by Shengyu's GSoC code that simply seemed to use fprintf for
>> debbugging, I tried adding some debug printing in inode.c
>>     fprintf (stderr, "gnu.translator[%d,%d]=%s\n", datalen, strlen (*namep), 
>> *namep);
> Printf is the simplest way to make sure things are happening the way one
> wants, yes. Note however that in the case of translators even the output on
> stderr is buffered, so you need to flush it with fflush(stderr). Even
> safer is to use snprintf + mach_print().
>> but that does not seem to work,
> More precisely?
> I'll assume "does not show up".

(that was more precisely exactly what I meant to ask)

> If your print doesn't show up, try to put a print in other places which
> are definitely to be called such as in diskfs_user_read_node(). If those
> come up, then it means the code you put your prints is not even called,
> so put prints in code you thought was calling it etc. up to the RPC that
> you thought would be called, then jump to libc which was supposed to be
> making the RPC call, etc.

Thank you!  It took me a while to find the right setfattr curse so I
dabbled in here some more and can confirm that combining your
instructions, e.g., like so

--8<---------------cut here---------------start------------->8---
diff --git a/ext2fs/inode.c b/ext2fs/inode.c
index a2e804b9..f4e29eb5 100644
--- a/ext2fs/inode.c
+++ b/ext2fs/inode.c
@@ -700,6 +700,9 @@ diskfs_get_translator (struct node *np, char **namep, 
unsigned *namelen)
   void *transloc;
   struct ext2_inode *di;
+  fprintf (stderr, "diskfs_get_translator\n");
+  fflush (stderr);
   if (sblock->s_creator_os != EXT2_OS_HURD)
     return EOPNOTSUPP;
--8<---------------cut here---------------end--------------->8---
"just work".  That's helpful knowledge to have anyway.


Jan Nieuwenhuizen <janneke@gnu.org> | GNU LilyPond http://lilypond.org
Freelance IT http://JoyofSource.com | AvatarĀ® http://AvatarAcademy.com

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